This invention relates in general to the casting of metal strip by continuous casting in a twin roll caster.
In a twin roll caster, molten metal is introduced between a pair of counter-rotated laterally positioned casting rolls that are internally cooled so that metal shells solidify on the moving casting surfaces and are brought together at a nip between the rolls to form a solidified strip product, delivered downwardly from the nip between the rolls. The term “nip” is used herein to refer to the general region at which the casting rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel or series of smaller vessels from which it flows through a metal delivery nozzle located above the nip, forming a casting pool of molten metal supported on the casting surfaces of the rolls immediately above the nip and extending along the length of the nip. This casting pool is usually confined between side plates or dams held in slidable engagement with end surfaces of the casting rolls against outflow.
Further, the twin roll caster may be capable of continuously producing cast strip from molten steel through a sequence of ladles. Pouring the molten metal from the ladle into smaller vessels before flowing through the metal delivery nozzle enables the exchange of an empty ladle with a full ladle without disrupting the casting campaign of cast strip.
The formed cast strip is delivered downwardly from the nip of the casting rolls into a first enclosure containing a protective atmosphere where the strip commences a cooling process after leaving the nip. After commencement of the campaign, the strip typically hangs in the enclosure in a loop before moving over a roller table through a first of pinch rolls. The strip then proceeds through the first set of pinch rolls typically to a roll mill to improve strip surface qualities, and to achieve desired thickness and mechanical properties, and thereafter to a cooling enclosure and a second set of pinch rolls for tension control, and then to a coiler.
The conditions provided for the just cast strip in the first enclosure influences the quality of the cast strip. We have found that microcracks in the surface of the finish cast strip can be traced to strain in the strip in the first enclosure as it moves through the loop and across the roller table into the first pinch rolls. Additionally, we have found that the oxygen and water vapor levels of the atmosphere in the first enclosure increase oxidation and scale formed on the strip and otherwise influence the surface quality of the cast strip. The present invention provides an apparatus for continuously casting thin strip having improved conditions for the strip after casting in the first enclosure.
Disclosed is an apparatus for continuously casting metal strip comprising:
a. a caster having a pair of casting rolls laterally positioned to form a nip there between capable of delivering cast strip downwardly from the nip,
b. a metal delivery system capable of forming a casting pool supported on casting surfaces of the casting rolls above the nip with side dams adjacent the ends of casting rolls to confine the casting pool, and
c. a first enclosure beneath the casting capable of forming a protective atmosphere into which the strip can be delivered from the casting rolls, and in which the strip can be formed in loop and extend over rollers into pinch rolls with the strip having a strain of less than 0.4%.
The loop in the first enclosure of apparatus for continuously casting thin strip may be between 2.5 and 4.5 meters in height from the nip.
The strain of less than 0.4% may be provided in the cast strip in the first enclosure by providing the plurality of rollers at entry of the strip into the pinch rolls to carry the strip, with at least a first entry roller having a diameter between 200 and 650 millimeters and being below a majority of other rollers. Alternatively, a strain of less than 0.3% may be provided in the strip in the first enclosure by providing the plurality of rollers at entry of the strip into the pinch rolls to carry the strip, with at least a first entry roller having a diameter between 200 and 650 millimeters and being below a majority of other rollers.
The apparatus for continuously casting thin strip may further comprise gas nozzles positioned adjacent to the entry of the cast into the first enclosure and capable of directing a gas towards the strip and inhibiting the flow of gas from the first enclosure upwardly to beneath the casting rolls.
The apparatus for continuously casting thin strip may comprise at least one additional enclosure between the casting rolls and a rolling mill through which the strip passes with an atmospheric pressure less than a pressure in the first enclosure and greater than the atmospheric pressure in a next proceeding enclosure. The atmospheric pressure in the first enclosure may be less than 0.5 inches on a water gauge, and the atmospheric pressure of each subsequent enclosure may be at least 0.03 inches less on a water gauge than an immediately preceding enclosure.
Further, the apparatus for continuously casting thin strip may further comprise a regulating device where the atmospheric pressure in each enclosure is maintained by a closed loop regulating device.
Various advantages of the present invention will become apparent from the following detailed description and accompanying drawings.